Wood is a widely used today as a building material for numerous outdoor applications, including decks, fencing, walkways, gazebos, and boardwalks. Wood is preferred to many other materials in these applications due to its appearance and relatively low cost. Wood is, however, a naturally porous organic material, and is highly susceptible to the growth of fungi and bacteria upon and within its exposed surfaces. Fungal and bacterial growth serves to discolor the wood and reduce its effective lifetime for these applications, since the growth of these microorganisms will eventually cause the wood to rot away. In addition, wood is susceptible to damage by insects such as termites. Given its porous structure, wood tends to absorb moisture, which furthers its breakdown over time. Finally, exposure to ultraviolet (UV) light over time causes the wood to fade and eventually break down. For all these reasons, untreated wood is generally not used in outdoor applications.
The art includes a number of methods of preserving wood that are intended to improve its lifetime when exposed outdoors, thus making it a more practical choice for various outdoor applications. One preservation solution is to simply paint the wood, such that its outer surface is no longer directly exposed. Paint, however, must be periodically re-applied, since even the most advanced paints will eventually wear, peel, blister, or otherwise lose their effectiveness. The periodic repainting required greatly increases the cost of wood as a building material for these applications. In addition, many people desire the appearance of unpainted, natural wood for applications such as decking and fencing, and thus painting the wood is not a feasible option for such applications.
Another method of preserving wood is by chemically treating the wood by various known means. The treatment of wood with tar has been known since ancient times. Railroad ties and utility poles are commonly pressure-treated using creosote as a preservative. A common process still used today to treat lumber involves the application of chromated copper arsenate (CCA). The CCA application process uses a vacuum and pressure cycle during which an aqueous solution of CCA is applied to the wood, after which the treated wood is stacked and left to dry. The application of pressure and vacuum cycles during the process causes a deeper and more uniform penetration of the wood by the applied preservative. The various components of CCA render different desirable properties to the treated wood; copper acts as a fungicide, arsenic acts as an insecticide, and chromium provides some measure of ultraviolet (UV) light resistance. CCA has fallen into disfavor, however, due to the concern that CCA may leech from wood over time, thereby causing environmental harm. As a result, alkaline copper quaternary (ACQ) has become more widely used as a preservative; this material is made up of copper, a quaternary ammonium compound, and an insecticide. Various other copper materials have also been used or proposed for use in connection with wood preservation.
More recently, particulate copper has begun to be used in wood preservation. In this case, copper is ground to micro-sized particles and suspended in water before being applied to the wood, rather than being dissolved in a chemical reaction such as with the CCA and ACQ processes. In order to be effective, the copper particles must be small enough to penetrate the porous wood structure. Reported sizes of such copper particles are on average less than 300 nm. Certain copper-treated wood products are advertised as utilizing micro-sized copper particle technology, such as MicroPro wood products marketed by Osmose, Inc., and Wolmanized wood products marketed by Arch Wood Protection, Inc. These products are simply wood lumber treated with the copper particles, and thus they are relatively expensive, and the degree to which the copper particles impregnate the wood is limited to only the very outer surface of the wood, while much of the wood remains untreated.
Due to the inherent difficulties of preserving wood and the cost of natural lumber, an environmentally sustainable solution to producing a more durable product with the appearance of wood is to form the product from wood-composite materials, which are in a class of materials known as engineered wood. For example, decking or fencing boards may be pressed from a blend of wood fiber and recycled plastic fibers. Such products are currently marketed by Trex Company as Transcend decking, and Advanced Environmental Recycling Technologies, Inc. as ChoiceDek composite decking. These materials may provide a cost advantage over wood lumber, since they can be formed of waste wood fiber and recycled plastics. In a typical process, the various component materials such as polymer, wood particles, coloring agents, antifungal agents such as boron or copper and iron based compounds, are simply mixed and blended in a vat, heated to a melting temperature, and then extruded into lumber-shaped pieces.
While composite decking materials may offer advantages over chemically treated wood lumber with respect to cost, they have not proven to be immune to mold and associated discoloration. Consumers have also reported the problem of “chalking,” whereby the material wears away in a powdery form on its surface, such that a person touching the material (as on a handrail or guard) may find his or her hand coated with a fine powder as a result. Wear may cause the product to quickly lighten in high-wear areas, resulting in an unsightly appearance. Other reported problems in these wood-polymer blends are UV discoloration, fungal growth, moisture trapping, and others. To stop such degradation, companies like Trex have begun producing a laminate wood-polymer composite with a polymer jacket. That polymer jacket, however, gets excessively hot, and is thus undesirable for many applications.
Because of the drawbacks in all of the available outdoor wood applications products on the market today, a new product that has the aesthetic appearance of wood but which is long-lasting and does not require continued cleaning and maintenance is highly desirable.
References mentioned in this background section are not admitted to be prior art with respect to the present invention.